In the development of electroluminescent devices utilizing organic materials (hereinafter referred to as organic EL device), the kind of electrodes has been optimized for the purpose of improving the electron-injecting efficiency from the electrode and a device in which a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex are disposed as thin films between the electrodes has been developed (Appl. Phys. Lett., Vol. 51, p. 913, 1987) to bring about a noticeable improvement in luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like. Following this, the developmental works of organic El devices have been focused on their commercial applications to high-performance flat panels characterized by self luminescence and high-speed response.
In order to improve the efficiency of such organic EL devices still further, various modifications of the aforementioned basic structure of anode/hole-transporting layer/light-emitting layer/cathode have been tried by suitably adding a hole-injecting layer, an electron-injecting layer, and an electron-transporting layer. For example, the following structures are known: anode/hole-injecting layer/hole-transporting layer/light-emitting layer/cathode; anode/hole-injecting layer/light-emitting layer/electron-transporting layer/cathode; and anode/hole-injecting layer/light-emitting layer/electron-transporting layer/electron-injecting layer/cathode. The hole-transporting layer has a function of transporting the holes injected from the hole-injecting layer to the light-emitting layer while the electron-transporting layer has a function of transporting the electrons injected from the cathode to the light-emitting layer.
The interposition of the hole-transporting layer between the light-emitting layer and the hole-injecting layer helps to inject more holes to the light-emitting layer by application of lower electrical field and, furthermore, the electrons injected into the light-emitting layer from the cathode or from the electron-transporting layer accumulate in the interface between the hole-transporting layer and the light-emitting layer as the hole-transporting layer obstructs the flow of electrons. As a result, the luminous efficiency improves.
Likewise, the interposition of the electron-transporting layer between the light-emitting layer and the electron-injecting layer helps to inject more electrons into the light-emitting layer by application of lower electrical field and, furthermore, the holes injected into the light-emitting layer from the anode or from the hole-transporting layer accumulate in the interface between the electron-transporting layer and the light-emitting layer as the electron-transporting layer obstructs the flow of holes. As a result, the luminous efficiency improves.
A large number of organic materials conforming to the function of these layered structures have been developed.
The aforementioned device comprising a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex and many others utilize fluorescence. Now, the utilization of phosphorescence, that is, emission of light from the triplet excited state, is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (singlet). To achieve this object, studies had been conducted on the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer, but the result was nothing but extremely low luminance. Thereafter, the use of europium complexes was attempted, but it failed to produce high luminous efficiency.
The prior technical documents relating to this invention are listed below.
Patent reference 1: JP2002-352957 A
Patent reference 2: JP2001-230079 A
Patent reference 3: JP2003-109765 A
Patent reference 4: JP2001-313178 A
Patent reference 5: JP2003-45611 A
Patent reference 6: JP2002-158091 A
Non-patent reference 1: Nature, Vol. 395, p. 151, 1998
Non-patent reference 2: Appl. Phys. Lett., Vol. 75, p. 4, 1999
The possibility of emitting red light at high efficiency by the use of a platinum complex (T-1, PtOEP) is reported in the aforementioned non-patent reference 1. Since then, it is reported in the non-patent reference 2 that the efficiency of emitting green light has been improved markedly by doping the light-emitting layer with iridium complexes (T-2, Ir(ppy)3). It is reported further that optimization of the light-emitting layer enables these iridium complexes to show extremely high luminous efficiency even when the structure of a device is simplified (Appl. Phys. Lett., Vol. 77, p. 904, 2000).
In applying organic El devices to display devices such as flat panel displays, it is necessary to improve the luminous efficiency and at the same time to secure the driving stability. However, the organic EL devices utilizing the phosphorescent molecule (T-2) described in the non-patent reference 2, although highly efficient, are not suitable for practical use because of their insufficient driving stability at the present time (Jpn. J. Appl. Phys., Vol. 38, p. L1502, 1999).
The main cause of the aforementioned deterioration of the driving stability is presumed to be the deterioration of the shape of thin film of the light-emitting layer in a device having a structure of substrate/anode/hole-transporting layer/light-emitting layer/hole-blocking layer/electron-transporting layer/cathode or substrate/anode/hole-transporting layer/light-emitting layer/electron-transporting layer/cathode. It is likely that the deterioration of the shape of thin film is attributable to crystallization (or cohesion) of thin organic amorphous films caused by generation of heat during driving of the device and low heat resistance is due to low glass transition temperature (Tg) of the material in use.
A carbazole compound (H-1, CBP) or a triazole compound (H-2, TAZ) is used in the light-emitting layer and a phenanthroline derivative (HB-1) is used in the hole-blocking layer in the non-patent reference 2. Because of their high symmetry and low molecular weight, these compounds readily undergo crystallization or cohesion and suffer deterioration in the shape of thin film. Besides, their crystallinity is too high to allow observation of the Tg. Such instability of the shape of thin film in the light-emitting layer adversely affects the performance of a device, for example, by shortening the driving life and lowering the heat resistance. For the reasons described above, a difficult problem facing phosphorescent organic electroluminescent devices at the present time is their driving stability.
It is disclosed in the patent reference 1 that a compound containing an oxadiazolyl group is used as a host material in an organic EL device comprising a host material and a phosphorescent dopant material in its light-emitting layer. An organic EL device comprising a thiazole or pyrazole structure in its organic layer is disclosed in the patent reference 2. An organic EL device comprising a pyrazole compound or a pyrazoline compound in its organic layer is disclosed in the patent reference 3. An organic EL device comprising a phosphorescent iridium complex and a carbazole compound in its light-emitting layer is disclosed in the patent reference 4. An organic EL device comprising a carbazole compound (PVK), a compound containing an oxadiazolyl group (PBD), and an iridium complex (Ir(ppy)3) in its light-emitting layer is disclosed in the patent reference 5. Ortho-metalated metal complexes and porphyrin metal complexes are proposed as phosphorescent compounds in the patent reference 6. However, they also face the aforementioned problem. It is to be noted that the patent references 2 and 3 disclose no organic EL devices utilizing phosphorescence.